Important Terms

  • atom - smallest piece of an element that keeps its chemical properties
  • compound - substance that can be broken into elements by chemical reactions
  • electron - particle orbiting the nucleus of an atom with a negative charge (mass = 9.10 x 10-28 grams)
  • element - substance that cannot be broken down by chemical reactions
  • ion - electrically charged atom (i.e., excess positive or negative charge)
  • molecule - smallest piece of a compound that keeps its chemical properties (made of two or more atoms)
  • neutron - particle in the nucleus of an atom with no charge (mass = 1.675 x 10-24 grams)
  • nucleus - dense, central core of an atom (made of protons and neutrons)
  • proton - particle in the nucleus of an atom with a positive charge (mass = 1.673 x 10-24 grams)

What is an Atom? The Legacy of Ancient Times Through the 19th Century

The modern view of an atom has come from many fields of chemistry and physics. The idea of an atom came from ancient Greek science/philosophy and from the results of 18th and 19th century chemistry:

  • concept of the atom
  • measurements of atomic mass
  • repeating or periodic relationship between the elements
Concept of the Atom

From the ancient Greeks through today, we have pondered what ordinary matter is made of. To understand the problem, here is a simple demonstration from a book entitled "The Extraordinary Chemistry of Ordinary Things, 3rd Edition" by Carl H. Snyder:

  1. Take a pile of paper clips (all of the same size and color).
  2. Divide the pile into two equal piles.
  3. Divide each of the smaller piles into two equal piles.
  4. Repeat step 3 until you are down to a pile containing only one paper clip. That one paper clip still does the job of a paper clip (i.e., hold loose papers together).
  5. Now, take a pair of scissors and cut that one paper clip in half. Can half of the paper clip do the same job as the single paper clip?

If you do the same thing with any element, you will reach an indivisible part that has the same properties of the element, like the single paper clip. This indivisible part is called an atom.

The idea of the atom was first devised by Democritus in 530 B.C. In 1808, an English school teacher and scientist named John Dalton proposed the modern atomic theory. Modern atomic theory simply states the following:

  • Every element is made of atoms - piles of paper clips.
  • All atoms of any element are the same - all the paper clips in the pile are the same size and color.
  • Atoms of different elements are different (size, properties) - like different sizes and colors of paper clips.
  • Atoms of different elements can combine to form compounds - you can link different sizes and colors of paper clips together to make new structures.
  • In chemical reactions, atoms are not made, destroyed, or changed - no new paper clips appear, no paper clips get lost and no paper clips change from one size/color to another.
  • In any compound, the numbers and kinds of atoms remain the same - the total number and types of paper clips that you start with are the same as when you finish.

Dalton's atomic theory formed the groundwork of chemistry at that time. Dalton envisioned atoms as tiny spheres with hooks on them. With these hooks, one atom could combine with another in definite proportions. But some elements could combine to make different compounds (e.g., hydrogen + oxygen could make water or hydrogen peroxide). So, he could not say anything about the numbers of each atom in the molecules of specific substances. Did water have one oxygen with one hydrogen or one oxygen with two hydrogens? This point was resolved when chemists figured out how to weigh atoms.